The invention pertains to 2-dimensional touch sensing surfaces operable by a human finger, or a stylus. Example devices include touch screens and touch pads, particularly those over LCDs, CRTs and other types of displays, or pen-input tablets, or encoders used in machinery for feedback control purposes.
In my earlier co-pending U.S. application Ser. No. 10/916,759, published as US 2005/0041018 A1, I describe a pattern of conductors which have anisotropic galvanic properties within the sensing region, due to the use of conductive stripes which prevent current flows in more than one direction, or possibly through the use of a special anisotropic conductive material which can be unpatterned. At least four electrodes are connected, one to each of the corners of the sensing layer and a capacitive sensing circuit which detects the signals associated with finger touch. A processor mathematically computes the centroid location of touch within the area using ratiometric methods. A simple quadratic equation or other method corrects for pin-cushion distortion that appears on only two sides of the sensing region.
In U.S. Pat. No. 3,593,115 there is shown a touch element having triangulated shapes for determining object position. However, this scheme requires numerous secondary electrode connections as well as two or more layers of construction, increasing construction costs and reducing transparency.
U.S. Pat. No. 5,650,597 shows a 2D sensing method which in its active area requires only one layer but requires large numbers of electrode connections. Resistive strips resolve one axis of position, and the accuracy is dependent on the tolerance of large numbers of resistive strips. This method however does suppress hand shadow effects.
In U.S. Pat. No. 6,297,811 there is described a touch screen using triangulated wire outline electrode shapes to create field gradients. However this patent suffers from the problem that it is difficult to scale up the screen size, as the number of electrode connections to a sensing circuit is one per triangle. It is desirable to dramatically reduce the number of connections in order to reduce cost and simplify construction. Also it is desirable to use solid shapes rather than wire outlines which are more expensive to construct. This method however does suppress hand shadow effects.
It is not known or obvious from the prior art how to combine any of the teachings from the above-mentioned patents in a way that provides for a one-layer sensing region with a reduced number of connection electrodes and suppression of hand-shadow as evidenced by the fact that to date nobody has arrived at such a solution.
The term ‘two-dimensional capacitive transducer’ or ‘2DCT’ will be used throughout to refer to touch screens, touch sensing pads, proximity sensing areas, display overlay touch screens over LCD, plasma, or CRT screens or the like, position sensing for mechanical devices or feedback systems, or other types of control surfaces without limitation, having a surface or volume capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
The term ‘two-dimensional resistive transducer’ or ‘2DRT’ will be used throughout to refer to touch screens or pen input devices based on purely galvanic principles, and known in the industry generically and primarily as ‘resistive touch screens’.
The term ‘2DxT’ refers to elements of either the 2DCT or 2DRT type.
The term ‘touch’ throughout means touch or proximity by a human body part or mechanical component of sufficient capacitive signal strength to generate a desired output. In the sense of ‘proximity’, touch can also mean to ‘point’ at a 2DCT without making physical contact, where the 2DCT responds to the capacitance from the proximity of the object sufficient to react properly.
The term ‘element’ throughout refers to the active sensing element of a 2DCT or 2DRT. The term ‘electrode’ refers to a connection point at the periphery of the element.
The term ‘stripe’ refers to an electrical line conductor that is a component part of an element and which has two ends. A stripe can be a wire. A stripe can have substantial galvanic resistance by intent, whereas a wire has minimal resistance. If the element of which it is a part is physically curved, the stripe would also be physically curved.
The term ‘pin cushion’ refers to any distortion of the signal from a 2DCT whether parabolic, barrel, or other form of 2D dimensional aberration.
Many types of 2DCT are known to suffer from geometric distortion characterized as ‘pin cushion’ or ‘hyperbolic’ or ‘parabolic’, whereby the reported coordinate of touch is in error due to electrical effects on the sensing surface. These effects are described in more depth in various other patents for example in Pepper U.S. Pat. No. 4,198,539. An excellent summary of the known causes, solutions, and problems of the solutions to geometric distortion can be found in a reading of Babb et al, in U.S. Pat. No. 5,940,065 and U.S. Pat. No. 6,506,983. U.S. Pat. No. 5,940,065 describes succinctly the two major classes of correction: 1) Electromechanical methods involving design of or modification to the sensing surface or the connecting electrodes; 2) Modeling methods using mathematical algorithms to correct the distortions.
In my U.S. Pat. No. 7,148,704 “Charge Transfer Capacitive Position Sensor” there is described in conjunction with FIG. 12 thereof a method of using individual resistive 1-D stripes to create a touch screen. These stripes can be read either in parallel or sequentially, since the connections to these stripes are independent of one another. Furthermore, in connection with FIG. 6 thereof there is described an interpolated coupling between adjacent lumped electrode elements and an object such as a finger. U.S. Pat. No. 7,148,704 is incorporated herein by reference.
In my U.S. Pat. No. 6,288,707 a capacitive position sensor is described that is intended to function as part of a computer pointing device that employs ratiometric capacitive sensing techniques. An array of patterned metallic electrodes is disposed on an insulating substrate layer, where the electrode geometry is selected to generate a varying capacitive output as a user's finger moves across the electrode array.
FIG. 7 of the accompanying drawings reproduces FIG. 4 of U.S. Pat. No. 6,288,707. An array of patterned metallic electrodes is disposed on an insulating layer where the electrode geometry is selected to generate a varying capacitive output as a user's finger moves across the electrode array. This arrangement comprises four interspersed electrode sets, two for each dimension. The x-axis sets, which are triangular, are easier to see and understand. A first set of triangles 1 are all electrically connected together to an output bus denoted as X1. The second set 2 are also connected together to an output labelled X2. The position of a user's hand with respect to the x-axis can be ascertained from the ratio of signals from X1 and X2. Because capacitance is directly proportional to surface area, and because the plates connected to X1 aggregate to a greater surface area to the left than do the plates connected to X2 (and vice versa) the ability to take the ratio of X1/X2 or X2/X1 is preserved so long as a great enough finger area is over the pattern at a close enough range to provide sufficient signal strength. A corresponding set of plates are connected to the Y1 and Y2 buses. The Y-connected set is also ratiometric, although in a manner different from the X sets. The Y set consists of alternating Y1-connected and Y2-connected rectangular stripes, 3 and 4 respectively, having a y-axis dimension that varies with placement in such a manner so as to create a smoothly varying ratio of surface area between Y1 and Y2 with location Y. The sum of each adjacent pair of the y-axis stripes 3 and 4 is made constant so that the sum of the capacitance is the same for any two paired stripes, i.e., C(Y1)+C(Y2)=C(Y) for each pair of stripes. Then, as the user's fingers move along the y-axis, the detected capacitance ratio is measured in the same manner as the CX1)/C(X2) ratio, i.e. the largest value becomes the numerator. However, this design offers limited resolution for the 2DCT.